Add "crossorigin" to boolean attributes

[fstanis]

An invalid keyword and an empty string will be handled as the anonymous keyword.

Source

[fstanis]

This is an interesting case, technically crossorigin="anonymous" can always safely be replaced by crossorigin, so that's an additional optimization that htmlnano could also do, but that'd require adding special logic just for this attribute, which is probably not worth it.

[maltsev]

Thanks for the PR!

[maltsev]

that's an additional optimization that htmlnano could also do, but that'd require adding special logic just for this attribute, which is probably not worth it.

I think it's worth it. I created a new issue for that: #68.